Preparations for tinting and coloring hair are an important type of cosmetic agent. They can be used to lighten or darken the natural hair color according to the wishes of the individual in question, to achieve a completely different hair color or to cover unwanted color tones, such as gray tones for example. Conventional hair coloring agents are formulated either on the basis of oxidation dyes or on the basis of substantive dyes, depending on the desired color and permanence. Combinations of oxidation dyes and substantive dyes are also often used to achieve special shades.
Coloring agents based on oxidation dyes lead to bright and permanent color tones. However, they require the use of strong oxidizing agents, such as hydrogen peroxide solutions for example. Such coloring agents contain oxidation dye precursors, so-called developer components and coupler components. Under the effect of oxidizing agents or oxygen in the air, the developer components form the actual dyes in conjunction with one another or by coupling to one or more coupler components.
Coloring agents based on substantive dyes are often used for temporary coloring. The substantive dyes are dye molecules which are absorbed directly onto the hair and require no oxidative process for developing the color. Important representatives of this class of dyes include for example triphenylmethane dyes, azo dyes, anthraquinone dyes or nitrobenzene dyes, each of which may have cationic or anionic groups.
In all these coloring processes, it may happen that the coloring is to be completely or partially reversed for various reasons. A partial removal of the color may be advisable for example if the color result on the fibers turns out to be darker than desired. On the other hand, a complete removal of the color may also be desired in some cases. For example, it is conceivable that the hair is to be colored or tinted in a particular shade for a specific event, and the original color is to be restored after a few days.
Various agents and methods for color removal are already known in the literature. One method for color reversal which is well known from the prior art is the oxidative decoloration of the colored hair, for example using a conventional bleaching agent. In this process, however, the fibers may be damaged due to the use of strong oxidizing agents.
Furthermore, reductive processes for color removal have also already been described. For example, European patent application EP 1300136 A2 discloses hair treatment methods in which the hair is colored in a first step and is reductively decolored again in a second step. The reductive decoloration takes place in this case by applying a formulation containing a dithionite salt and a surfactant. In WO 2008/055756 A2, the reductive decoloration of keratin fibers is carried out using a mixture of a reducing agent and an absorbent.
In documents WO 2012/069599, WO 2014/174230 and WO 2013/017862, various sulfinic acid derivatives are described in agents for reductively removing color from colored hair. However, the presentation form and decoloration result of the agents described in said documents cannot yet be referred to as optimal.
When using reductive decoloring agents, the decoloration takes place by reduction of the dyes on the keratin fibers or hair. Through the reduction, the dyes are usually converted into their reduced leuco forms. During this procedure, the double bonds present in the dyes are reduced, the chromophoric system of the dyes is in this way interrupted, and the dye is transformed into a colorless form.
For reducing the dyes, usually strong reducing agents must be used which may enter into undesired reactions with oxidizing agents, such as oxygen in the air for example. In aqueous solution, the reducing agents are moreover often not very stable and are broken down at greater or lesser speed depending on the pH of the solution. For example, the reductive decoloring agent sodium dithionite, which is known from the prior art, is sensitive to atmospheric oxygen and slowly breaks down in aqueous solution. By increasing the pH, these breakdown reactions can be delayed. Adjustment to a weakly alkaline pH stabilizes aqueous dithionite solutions, so that the solution can be stored for several weeks to months if oxygen is excluded. However, if the reductive decoloring agents are to be stored for longer, if the exclusion of air cannot be reliably ensured or if high temperatures prevail under storage conditions, then presentation in solution, in particular in a water-containing solution, is not the method of choice. In order to circumvent this problem, often the reducing agents themselves are used as a solid, for example in powder form, in the prior art documents. However, this procedure is associated with various disadvantages.
The reducing agents must be dissolved in a cosmetic carrier before being applied. If, in this regard, they are incorporated in pure form, for example as a powder, in the cosmetic carrier, dust may arise which, if inhaled, may irritate the user's airways. If the particle sizes of the pulverulent reducing agent are selected to be larger, there is the risk that the reducing agents will not sufficiently dissolve and thus may lead to an uneven, unattractive decoloration result. If not fully dissolved, the reducing agent is also not fully available for the decoloring process, so that the decoloration result may in this case turn out to be weaker than planned.
If the solubility of the particulate reducing agent in the cosmetic carrier is poor, the user is moreover forced to mix together the solid reducing agent and the cosmetic carrier for a very long time. For the user, this procedure is inconvenient, time-consuming and therefore largely undesirable.
It is therefore desirable to provide a decoloring agent for decoloring colored keratin fibers, which decolors colored keratin fibers as evenly and as effectively as possible. The decoloring agent should have a high degree of storage stability and a good decoloring performance even after long storage periods at high temperatures. The ready-to-use decoloring agent should be easily prepared and conveniently applied by the user. In particular, no dust should arise during application. In addition, the consistency of the decoloring agent should be optimized so that on the one hand it can be easily applied to and distributed on the head of the user, but on the other hand does not drip down from the keratin fibers. Finally, the decoloring effect should be improved in comparison to the decoloring agents known from the prior art.
Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with this background of the invention.